Add comprehensive ANN benchmarking suite

Extend benchmarks-ann/ with results database (SQLite with per-query detail
and continuous writes), dataset subfolder organization, --subset-size and
--warmup options. Supports systematic comparison across flat, rescore, IVF,
and DiskANN index types.

benchmarks-ann/datasets/cohere1m/Makefile

@@ -0,0 +1,24 @@
+BASE_URL = https://assets.zilliz.com/benchmark/cohere_medium_1m
+
+PARQUETS = train.parquet test.parquet neighbors.parquet
+
+.PHONY: all download base.db clean
+
+all: base.db
+
+download: $(PARQUETS)
+
+train.parquet:
+	curl -L -o $@ $(BASE_URL)/train.parquet
+
+test.parquet:
+	curl -L -o $@ $(BASE_URL)/test.parquet
+
+neighbors.parquet:
+	curl -L -o $@ $(BASE_URL)/neighbors.parquet
+
+base.db: $(PARQUETS) build_base_db.py
+	uv run --with pandas --with pyarrow python build_base_db.py
+
+clean:
+	rm -f base.db